1. Field of the Invention
The present invention relates to the field of wireless communications.
2. Description of Related Art
In contrast to Time Division Multiple Access (TDMA) and Frequency Division Multiple-access (FDMA) techniques, which create multiple communication channels from a single radio frequency (RF) bandwidth by assigning different time slots to mobile subscriber terminals (“mobiles”) and subdividing an RF band into a plurality of sub-bands respectively, systems which are based on spread spectrum techniques, such as Code Division Multiple Access (CDMA) systems, exhibit “soft capacity” by using orthogonal code sequences to differentiate mobiles. In other words, the number of mobiles that a single cell/sector of a CDMA system can support at one time is not fixed, and instead is generally limited only by the degradation of service quality caused by interference from other mobiles in the same or adjacent cells/sectors.
To address this tradeoff between network capacity and service quality, CDMA system architectures typically utilize reverse link, i.e., mobile to base station, power control techniques by which the base station adaptively sets the transmit power of each mobile being served to the minimum level required to maintain adequate performance (usually assessed by comparing the ratio of energy per bit, Eb, to interference, No, at the base station with a target Eb/No value). As interference at a network base station increases with increased reverse link load levels (hereinafter “load levels”), the base station issues mobile transmit power up-adjust commands as needed. At high load levels, the substantial interference which is likely to occur at the base station prompts the base station to issue an increased number of power up-adjust commands, particularly to those mobiles at outer cell/sector boundaries, thereby resulting in even greater interference at the base station. If not otherwise addressed, such increases in interference ultimately result in loss of base station coverage area (i.e., cell/sector shrinkage) because distant mobiles will not be able to transmit at the power level needed to achieve adequate call quality. Therefore, calls from such distant mobiles may be dropped under high load conditions.
To protect against such instability and loss of base station coverage area, CDMA networks commonly rely on call admission schemes, whereby mobiles in a heavily loaded cell/sector may be denied service from the corresponding base station. Assuming a static environment, the maximum number of users, Nmax, that can be served in a CDMA cell/sector (i.e., a 100% load level) can be expressed as:
                                          N            max                    =                                    PG                              v                ⁢                                                      E                    b                                                        N                    o                                                                        ×                          1              β                                      ,                            (        1        )            where PG is the processing gain of the CDMA system and is defined as the ratio of bandwidth used to the data rate achieved, ν is the voice activity, and β is the reuse efficiency of the CDMA cellular approach and is defined as the ratio of interference from other cells/sectors to interference within the cell/sector. When the cell/sector serves N users, the load level can be expressed as:
                    L        =                              N                          N              max                                =                                                    Nv                ⁢                                  Eb                  No                                            PG                        ⁢            x            ⁢                                                  ⁢                          β              .                                                          (        2        )            Measured Eb/No, voice activity ν, and CDMA reuse efficiency β are typically varying quantities, however. In particular, feasible approaches for accurately measuring β are unknown, and, thus, the above expression cannot be used in practice to determine load levels.
One current approach calculates load levels as a function of the ratio of total receive power rise measured at the base station to background noise. More specifically, as set forth in R. Padovani, Reverse Link Performance of IS-95 Based Cellular Systems, IEEE Personal Communications, pp. 28–34, 1994, there is a direct relationship between load levels and the ratio of total received power at the base station to background noise, which may be expressed as:
                              L          =                      1            -                          1              Z                                      ,                            (        3        )            where Z is the ratio of total receiver power to background noise. Background noise includes thermal noise as well as other non-CDMA interference such as jammer signal power. A drawback of this approach, however, is the difficulty of obtaining an accurate measure of background noise, and in particular thermal noise, in a dynamic network environment, and therefore accurate reverse link load level calculations utilizing the above expression cannot typically be realized.